Most editors don't know what alt text is, and even fewer know how to write it. It would be helpful if basic alt text were included in Commons, and, therefore, it would be helpful if a place for multi-lingual alt text were built into any structured data system that supports Commons.
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Note on alt text: This is for the benefit of people using Commons, i.e., when there is no contextual consideration, but where "Painting of X" or "Headshot of Joe Film" might be helpful to sort through images with sometimes unreadable names.
Wikipedia editors might or might not choose to copy the basic alt text, but if they did, they could easily change it whenever "Painting of X" or "Headshot of Joe Film" was not the appropriate description in their context.
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The Wikidata "alt" property proposal is stalled, but an "alt" attribute is basically how you would do it. Structured data can handle that. (I don't know if structured data is meant to be included in Wiki* via Lua/magic word or used only on Commons.)
This task is probably a resolved duplicate, I simply don't know where to point it.
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Above said, that property proposal is interesting reading, so @Whatamidoing-WMF might consider reviewing it for context.